A quality replacement job or new construction job starts with a proper load calculation. Room sizes, ceiling heights, house orientation to the sun exposure, windows, and doors along with insulation, people, and appliances are entered into a computer program to determine exactly what size system is needed for your home. The old rule of thumb based on square footage is not an accurate means of sizing equipment, especially in todays energy efficient homes. Two homes of they same size may need different sized equipment based on how the homes face the morning and evening sun.
Always request that a computer-generated load calculation be done on your home before getting a proposal. This is also needed to do a proper duct design and airflow calculations. Without the proper duct design the system will not work to the best of its ability. The ductwork and equipment work together to give you a complete system to provide comfort for your home. You cannot have one without the other. Get the facts and not the guesses.